Abrasive jet machining may be defined as the removal of material through the reaction of a focused, high velocity stream of fine grit or powder-loaded air. Basic apparatus for carrying out the abrasive jet machining process may include an air or gas supply, a pressurized powder reservoir with cooperating vibrating means, a mixing chamber, and a conduit for feeding the resultant powder to a hand held piece having an appropriate nozzle for directing the powder-air mixture against a workpiece for delicate removal of portions thereof. A foot or remote switch may control the air pressure at the nozzle. The pulsation of the vibrating means or the positive pressure within the powder reservoir urges the powder through a nozzle orifice or passageway to contact a pressurized air stream or jet. The amount of powder delivered is dependent upon the amplitude of vibration, pressure of the delivery system, diameters of the handpiece nozzle and power feed supply orifice or passageway, and size of powder, among other factors.
The abrasive powder, such as aluminum oxide and silicon carbide, for example, should be well classified and clean, and typically ranges in size from about 10 to 150 microns in diameter.
Abrasive jet machining is not a mass material removal process, but one of finishing. Because of the small amount of abrasive powder flowing through the handpiece nozzle at any given instant, it is not difficult to remove selected portions of the workpiece.
Current powder throttling mechanisms have a tendency to produce unreliable powder flow rates resulting in an excess or deficiency of grit or abrasive powder particles in the powder-air mixture and do not automatically adjust powder flow rate to maintain an acceptable or desirable proportion of powder in the entrained air powder stream or mixture.
The present invention provides a powder metering apparatus having an improved powder throttling mechanism for precisely metering powders for use in abrasive jet machining processes and automatically adjusts powder flow to provide proper proportions of entrained powder in the air-powder mixture.
More specifically, when powder flow rates are excessive for a given handpiece nozzle diameter, clogging of the nozzle frequently results. Conversely, an inadequate powder flow rate results in a decrease of cutting or removal efficiency. Because the powder delivery rate of the powder throttling mechanism of the present invention is dependent in large part upon the air velocity passing through the gap, which velocity at a given pressure is dependent upon diameter of the handpiece nozzle, the ratio of entrained powder to air mixture remains substantially constant, thereby preventing the aforementioned excesses or inadequacies.
The apparatus is substantially cylindrical, measures about 6" high with a 4" diameter, making it readily portable, and is reliable, inexpensive to fabricate and maintain, requires no electrical power source of any kind, and its throttling mechanism is readily adjustable to accommodate all powder flow rates employed in conventional abrasive jet machining operations. The apparatus requires a pneumatic power source for driving a ball around a raceway to produce controlled vibrations, and for issuing a continuous jet stream of air to be directed at the gap, i.e., the space between the outlet of the powder feed supply orifice and the throttling "needle", which needle must be substantially flat at its uppermost or tip portion. If the throttling needle is made conventional to project into the powder feed orifice, clogging of powders at the annular gap formed thereby would be commonplace.